This invention relates to method and apparatus for synthesizing filamentary structures including nanotubes in the post-flame region of a non-sooting premixed or non-premixed flame using unsupported catalysts.
Since their discovery in 1991(1), carbon nanotubes have sparked a surge of interest(2-5). Numbers in parentheses refer to the reference list included herein. The teachings in all of these references are incorporated by reference herein. The many unique properties of nanotubes gives appeal to a wide range of potential applications in areas such as mechanical actuators(6,7), sensors(8-10), polymer composites(11), electronics(12-16), biosensors and biocompatibility(17,18), gas storage(19-22), adsorption(23-25), and catalysis(26-28). Techniques that have been demonstrated to synthesize carbon nanotubes include laser ablation(29,30), plasma arc(31), chemical vapor deposition (CVD)(32-34), fluidized bed reactors(35,36), and combustion systems(37-51).
Flames offer potential as means of producing bulk quantities of carbon nanotubes in a continuous, economically favorable process. There are three key requirements for nanotube synthesis common to most of the synthesis techniques: 1) a source of carbon, 2) a source of heat, and 3) presence of metallic catalyst particles. A fuel-rich flame is a high-temperature, carbon-rich environment that can be suitable for nanotube formation if certain metals are introduced into the system.
There have been a number of reported observations in the combustion literature of nanotubes and filamental carbon structures within flame systems. Perhaps the earliest observation of intriguing tube-like structures in flames is reported by Singer(52) in the 1950s and within the last decade there have been occasional reports of nanotube structures (38,49,50,53-57). These observations are typically reported as curiosities and are largely serendipitous in nature. In recent years Diener et al.(51), Saito et al.(49,50), and Vander Wal et al(37-45) have independently made more detailed studies of nanotube formation in flames.
Diener et al.(51) report the synthesis of single-walled carbon nanotubes in sooting flames. A semi-premixed flame configuration is used with fuel gases (acetylene, ethylene or benzene) issued through numerous small diameter tubes distributed through a sintered metal plate through which oxygen flows, drafting past the fuel tubes. Iron and nickel bis(cyclopentadiene) compounds are vaporized and issued to the flame feed as a metal catalyst precursor. Single-walled carbon nanotubes are observed in acetylene and ethylene flames (within the equivalence ratio range of 1.7 to 3.8) while multi-walled nanotubes are observed in benzene flames (within the equivalence ratio range of 1.7 to 3.4). An equivalence ratio, φ, is defined as the actual fuel/oxygen ratio divided by the stoichiometric fuel/oxygen ratio corresponding to conversion of all carbon to CO2 and all hydrogen to H2O. Diener et al. do not report the level of dilution with argon, the concentration of metal species added to the flame, or the inlet velocity for the feed gas mixture—all are parameters affecting nanotube formation in flames. The reported overall single-walled carbon nanotube yields are very low “certainly less than 1% of the carbon soot product” and this small population of single-walled carbon nanotubes is confirmed by inspection of the transmission electron microscope (TEM) micrographs in the article. The nanotube bearing soot material analyzed by Diener et al. is collected from a filter system far downstream from the burner and there is no information relating to the time, temperature or concentration history of the material, making it difficult to judge the extent to which nanotubes were in fact formed in the flame and how much growth occurred during extended exposure to flame exhaust while collecting in associated downstream systems. Prior to analysis, the material is prepared using a separation technique employing sonication of the soot in methanol to disperse the sample—it is unclear how this preparation technique might alter the composition of the material and if the material is representative of solid material present in the flame itself. Diener et al. place emphasis on the use of sooting flames for the synthesis of their materials which is in fact analogous to the approach reported by Howard et al.(46-48), Richter et al.(55), and Duan et al. (54). The reported range of equivalence ratios is stated as 1.7 to 3.8 which is very much focused on exploiting sooting conditions. Furthermore, the quantities of nanotubes observed in the condensed material is very small (<1%).
Saito et al.(49,50) immersed metallic substrates in various hydrocarbon fueled diffusion flames and observed multi-walled carbon nanotubes that had formed on the substrate. It will be understood by those familiar with the combustion literature that a diffusion flame is one type of non-premixed flame. Vander Wal and coauthors have observed single walled nanotubes in a hydrocarbon (acetylene or ethylene)/air diffusion flame with nitrogen diluent and metallocene catalyst precursor compound added to the fuel stream (38).
Vander Wal and coworkers make extensive use of an annular burner configuration consisting of a 50 mm diameter sintered metal plate with a central tube of 11 mm diameter that is mounted flush with the surface of the burner plate. See FIG. 1. For most experiments Vander Wal et al. established a fuel rich premixed flame supported on the outer annular section of a burner plate 10 while reactant gas mixtures, including metal catalyst species of interest were fed through a central tube 12. This configuration is termed a ‘pyrolysis flame’ in the papers as the central gas flow does not undergo combustion due to the lack of oxygen in this flow, but reactions (and nanotube formation) do proceed in the flow by virtue of the heating influence of the surrounding annular flame. The central gasflow is in effect a reactive streamtube and not a flame. A stabilizing chimney 14 (7.5×2.5 cm diameter) immersed in the flame gases provides a stabilizing effect and nanotube (single-wall, multi-wall nanotubes and nanofiber) samples are collected at the exit of the chimney. There are some important distinctions to note regarding this configuration. First of all, the outer (annular) flame is primarily a source of heat and the central gas mixture flow is the primary source of carbon and metallic catalyst. Combustion is not supported in the central gas flow. Therefore, heating and material synthesis processes are substantially separated functions.
A flame system has been used extensively in combination with a wide variety of methods to introduce metallic catalyst species to the system. Vander Wal and Ticich performed comparative experiments, synthesizing nanotubes in both the ‘pyrolysis flame’ and tube reactor setups(39,40). The premixed flame in the outer annulus used acetylene/air mixtures of equivalence ratios between 1.4 and 1.62. The reactant gas mixtures used in this instance used either carbon monoxide/hydrogen or acetylene/hydrogen mixtures, and iron or nickel nanoparticles entrained in the central feed gases. In a similar study, Vander Wal and Ticich used a carbon monoxide/hydrogen reactant feed mixture and used a nebulized solution of iron colloid (ferrofluid) and a spray drying technique as the source of catalyst particles. Nanotube samples were collected once again at the exit of the chimney section(39). Single-walled nanotubes were observed in a similar flame setup where Vander Wal and Hall introduced metallocene (ferrocene and nickelocene) vapor to the central reactive feed gases using a controlled sublimation technique(45). Vander Wal observed single-walled nanotubes in an identical flame arrangement using a nebulizer system to introduce iron nitrate salt solution to the flame as the catalyst particle precursor(37). Vander Wal also reports the formation of nanofibers (similar to multi-walled nanotubes except the walls tend to be irregular and non-graphitic) in an identical flame configuration with nickel nitrate solution nebulized into the flame(44).
Another variation of the catalyst feed technique with this burner configuration is reported by Vander Wal, where catalyst particles are generated by burning a piece of paper coated in metal particles and the resulting aerosol is entrained in a fuel-rich mixture of carbon monoxide, hydrogen and air. The resulting gas mixture is fed to the central section of an annular fuel-rich acetylene-air flame and single-walled nanotubes are collected at the exit of a cylindrical chimney surrounding the central streamtube. In this instance the central gas flow does in fact lead to a premixed flame (as opposed to a pyrolysis reaction streamtube in previous experiments) where the premixed flame composition is carbon monoxide, hydrogen and air with entrained iron nano-particles. Single-wall nanotubes were once again collected at the exhaust of the stabilizing chimney(43). In this configuration, the premixed gas feed did not contain a hydrocarbon (carbon monoxide and hydrogen are used in this case). Further, the nanotube material is collected quite late in the system at a point exclusively at the exhaust of a physical chimney insert.
Vander Wal, Hall, and Berger have synthesized multi-walled nanotubes and nanofibers on cobalt nanoparticles supported on a metal substrate immersed in premixed flames of various hydrocarbon fuels and equivalence ratios(41,42). This configuration is truly a premixed flame and all three functions necessary for nanotube synthesis (heat source, carbon source, and metal catalyst) are present in the same flame environment. However, in this instance the catalyst particles are supported on an externally affixed substrate immersed in the flame gases.
An extensive amount of research related to the formation of fullerenes and fullerenic nanostructures in flames has been reported in the last decade(46-48,56,58,59). In particular, there have been two studies by Howard et al. where carbon nanotubes have been observed in condensed material collected from flames(47,48). Howard et al. employed a premixed flame configuration operated at low pressure (20 to 97 Torr), and burner gas velocity between 25 and 50 cm/s. A variety of fuels and fuel/oxygen compositions (C/O ratios) were explored including acetylene (C/O 1.06, φ=2.65), benzene (C/O 0.86 to 1.00, φ=2.15 to 2.65) and ethylene (C/O 1.07, φ=3.21). Diluent concentrations between 0 and 44 mol % were also explored. These flames are all considered ‘sooting’ flames as they spontaneously generate condensed carbon in the form of soot agglomerates suspended in the flame gases. Similarly, other studies that have reported nanotubes in flames such as Duan et al.(54) and Richter et al.(55) have each been under sooting conditions. Samples of condensed material were obtained directly from the flame using a water-cooled gas extraction probe (between 2 to 7 cm above burner), and also from the water-cooled surfaces of the burner chamber. Nanostructures were extracted from the collected soot material by sonication of soot material dispersed in toluene. High resolution electron microscopy of the extracted material allowed visual analysis of the fullerenic nanostructures. A range of nanostructures was observed, including spherical, spheroidal, tubular and trigonous structures, typically composed of multiple, graphitic carbon planes. Nanotubes are observed in these materials and tend to be multi-walled nanotubes typically with more than 5 walls. The nanotube material is generally observed predominately in the material collected from the chamber surfaces. U.S. Pat. No. 5,985,232 has been awarded for ‘production of fullerenic nanostructures’ that draws heavily on the methods and observations reported in the papers described above(46). The patent discloses a method based on a flame burning unsaturated hydrocarbons, operated at sub-atmospheric pressure (up to 300 Torr), with diluent present in the flame feed gases, and also makes allowance for the addition of metal species (such as iron, cobalt, nickel, calcium, magnesium, potassium, rubidium and strontium) to promote the formation of single-walled nanostructures. Additional disclosure relates to the potential of adding oxidant species to the flame gases to selectively purify the nanostructures relative to the soot material and possibly open the end-caps of nanotube materials.
There have been a number of combustion studies that have employed some components of the system described in the present patent application, yet did not observe the formation of carbon nanotube material. Rumminger et al. (60,61) introduced a vapor of iron pentacarbonyl into premixed flames of methane/air and also carbon monoxide/hydrogen/air. The focus of the studies was on flame inhibition due to the compound. No nanotube material is reported from this work and the likely reason is the low equivalence ratio employed in these studies. Feitelberg and coworkers also injected metal compounds into premixed flames in order to examine the effect upon soot formation in fuel rich flames. Nanotube-like material was not reported from these studies, most likely because the equivalence ratios employed were too high. Janzen and Roth (62) examined the formation of iron-oxide particles in a premixed hydrogen/oxygen/argon flame injected with iron pentacarbonyl and did not observe any nanotube formation. The reason is very simply that there was insufficient carbon in this flame system. Each of these flame studies employed some, but not all, of the components that have been found to favor nanotube formation in a premixed flame.